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The researchers looked at almost 6,500 Taiwanese men over 13 years of age. They found that for every 5 microgram increase in concentrations of fine particulate matter – called PM2.5 – there was a 1.29 percent decrease in the amount of normal-shaped sperm.

“Air pollution has been associated with a variety of adverse effects on humans. PM2.5 have been associated with lung and heart disease. So this report that that fine particulate matter exposure results in reduced normal sperm morphology and sperm movement, which may impact male fertility, is plausible.”

However, the study wasn’t able to draw a definite line between PM2.5 and less healthy sperm.

As Professor Kevin McConway from The Open University in the UK pointed out, “the researchers did not take into account other air pollutants, and many such pollutants tend to go together. So if there really is an effect on sperm characteristics, maybe it could be caused by different pollutants.”

“In any case, this is an observational study, so it can’t be clear on what might cause what.”

Despite the possibility that other pollutants might be involved, one of the strengths of the study was that the researchers had lots of detail about the men, and they used satellite monitoring to accurately pinpoint the precise levels air pollution at each man’s home. The reduction in normal-shaped sperm was pretty consistent, even after taking into account factors such as age, weight and alcohol consumption.

Whether the consistent but small decrease in normal-shaped sperm would apply to other men in other locations isn’t certain. Air pollution can vary from place to place quite dramatically, and Taiwan experiences quite a lot more air pollution than most Australian cities. According to World Health Organization data, the average PM2.5 concentration in Taiwan is 19 micrograms per cubic metre, whereas Australian cities average only 6–8 micrograms.

Still, according to Dr Musgrave, the average concentration measured in the study was “not dissimilar to the highest daily average PM2.5 concentrations seen in Australian capital cities.”

So should men be worried that air pollution is messing with their fertility? There are good reasons not to be too concerned. One surprising finding from the study was that sperm concentrations increased, slightly, as PM2.5 levels rose.

“This makes no sense to me,” said Professor Allen Pacey from the University of Sheffield in the UK, “and does not fit with any biological mechanism that I am aware of.”

“I remain of the opinion that air pollution probably does have the potential to negatively influence male reproductive health. But the jury is still out about quite how and to what extent this impacts on male fertility.”

The authors suggest that if air pollution really is impacting sperm health, even a small effect could tip a “significant number of couples” into infertility, simply because so many people are exposed to dirty air.

For Professor McConway, that isn’t enough to ring alarm bells.

“If I were young enough to worry about my fertility, I wouldn’t put moving to an area with cleaner air at the top of my list of actions – though there are certainly many other health-related reasons to live in cleaner air.”

A pair of new studies conclude that there is not yet enough evidence to decide whether – and for which patients – marijuana is an effective treatment for chronic pain or post-traumatic stress disorder.